A ferroelectric has spontaneous polarization, and the polarization direction can be controlled. For example, a ferroelectric memory utilizes the variability in spontaneous polarization of a ferroelectric. When a voltage is applied to the ferroelectric, the direction of the spontaneous polarization is changed, and electric charge moves according to this change. These properties of the ferroelectric can be used to form an electric switch, and the ferroelectric memory including the electric switch serves as a nonvolatile memory. It is known that a change in conductivity due to a change in spontaneous polarization can cause a leakage current in the ferroelectric memory. Therefore, a method or the like for preventing the change in conductivity has been studied to deal with the leakage current. A technique of using the leakage current also has been proposed (see JPN 10 (1998)-56141 A). Moreover, as an example of using the polarization of a ferroelectric, an electric switch has been proposed that utilizes a change in electric conductivity caused by an overcurrent.
Regarding the polarization of a ferroelectric, there has been a report that domain-inverted regions of an X-plate Mg-doped LiNbO3 exhibit rectification properties and decrease in resistance (see S. Sonia, I. Tubular, and M. Hater; Applied Physics Letters, vol. 70, pp. 3078-3079, 1997).
Another report has shown an electric switch that changes the electric conductivity by allowing an overcurrent to flow through the ferroelectric (see Y Waianae, J. G. Boxer, A. Bisects, Co. Gabber, D. Wider, A. Beck; Applied Physics Letters, vol. 78, pp. 3738-3740, 2001).
As described above, there are various techniques to use the polarization of a ferroelectric. In the ferroelectric memory, the insulating properties are reduced due to a change in spontaneous polarization. This phenomenon may degrade the characteristics of the ferroelectric memory. The degradation of the characteristics means that the conductivity is changed by approximately several to several tens of times.
The advantages of stability, mass-productivity, and reliability in commercially available ferroelectric memories that utilize a change in spontaneous polarization of a ferroelectric have been demonstrated by their wide application. However, when an electric field is produced by the movement of electric charge according to a reversal of the spontaneous polarization, the conventional ferroelectric memories only use the electric field indirectly as a voltage to drive a semiconductor switch. Therefore, the configuration is complicated, and the degree of integration is restricted. Moreover, the conventional ferroelectric memories are not sufficient, e.g., for the memory life, the number of repeated switching operations, and the time for storing nonvolatile charge. On the other hand, attempts are being made to use a change in spontaneous polarization directly for switching. However, it has not been accomplished yet because appropriate ferroelectric materials are not found.